Magnetic storage apparatuses typified by magnetic disk drives include a built-in type which is built into a personal computer (PC) or the like, and an external type which is connected externally to the PC or the like. In such magnetic storage apparatuses, there are demands to increase the surface recording density of the magnetic recording medium in order to increase the storage capacity of the magnetic storage apparatus.
Recently, magnetic storage apparatuses employing the perpendicular magnetic recording technique, having a high recording bit stability even at high recording densities, have been reduced to practice. When increasing the surface recording density of the magnetic recording medium, it is necessary to reduce the medium noise in the perpendicular magnetic recording medium which employs the perpendicular magnetic recording technique, similarly as in the case of the magnetic recording medium employing the horizontal (or in-plane) magnetic recording technique.
A proposal has been made to use a granular magnetic layer for a perpendicular recording layer of the perpendicular magnetic recording medium in order to reduce the medium noise. In the granular magnetic layer, a nonmagnetic material, such as an oxide or a nitride, is formed at a grain boundary of magnetic grains to magnetically separate or isolate the magnetic grains, in order to reduce the medium noise. In addition, various methods have been proposed to promote the magnetic separation or isolation of the magnetic grains in the granular magnetic layer. For example, a Japanese Laid-Open Patent Publication No. 2005-353256 proposes a method of separating crystal grains of a nonmagnetic intermediate layer immediately under the perpendicular recording layer by a gap.
In order to further increase the surface recording density of the perpendicular magnetic recording medium, it is necessary to further improve the signal-to-noise ratio (SNR). Presently, the mainstream measures for further improving the SNR reduces the medium noise. In order to reduce the medium noise, it is necessary to reduce the magnetic grain size, make the magnetic grain size uniform, and reduce the crystal orientation dispersion of the perpendicular recording layer. In this specification, the reducing of the crystal orientation dispersion of the crystal grains such as the magnetic grains will be referred to as achieving high (or improved) orientation. The nonmagnetic intermediate layer provided immediately under the perpendicular recording layer plays an important role in determining the magnetic grain characteristics described above. If the reduction of the grain size, reduction of the crystal orientation dispersion and the high orientation can be achieved with respect to the crystal grains of the nonmagnetic intermediate layer, it would be possible to reduce the grain size, reduce the crystal orientation dispersion and achieve the high orientation with respect to the perpendicular recording layer that is epitaxially grown on the nonmagnetic intermediate layer.
A technique has been proposed to provide a seed layer immediately under the nonmagnetic intermediate layer in order to reduce the grain size, reduce the crystal orientation dispersion and achieve the high orientation with respect to the nonmagnetic intermediate layer. For example, a Japanese Laid-Open Patent Publication No. 2007-179598 proposes providing a NiW seed layer having a face centered cubic (fcc) structure immediately under a Ru intermediate layer having a hexagonal close packed (hcp) structure. However, according to the medium structure proposed in the Japanese Laid-Open Patent Publication No. 2007-179598, reducing the crystal grain size of the seed layer and achieving the high orientation of the crystal grains of the seed layer are in a tradeoff relationship, and there was a limit to simultaneously reducing the crystal grain size and achieving the high orientation of the seed layer. For this reason, there was a limit to simultaneously reducing the crystal grain size of the perpendicular recording layer and achieving the high orientation of the perpendicular recording layer. Consequently, there was a limit to further reducing the medium noise in the conventional perpendicular magnetic recording medium, and there was a limit to further increasing the surface recording density of the conventional perpendicular magnetic recording medium.
The applicants are also aware of Japanese Laid-Open Patent Publications No. 2001-155321, No. 2007-257804 and No. 2007-184019. The applicants are further aware of Toshio Ando et al., “Triple-Layer Perpendicular Recording Media for High SN Ratio and Signal Stability”, IEEE Transactions on Magnetics, Vol. 33, No. 5, September 1997, pp. 2983-2985, and S. S. P. Parkin, “Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3d, 4d, and 5d Transition Metals”, Physical Review Letters, Vol. 67, No. 25, Dec. 16, 1991, pp. 3598-3601.
Therefore, in the conventional perpendicular magnetic recording media, there were problems in that there is a limit to simultaneously reducing the magnetic grain size and achieving the high orientation of the perpendicular recording layer, and that it is difficult to further reduce the medium noise.